1. Field of the Invention
The present invention relates to methods and systems for reducing the memory required in digital video broadcasting (DVB-T/H) receivers.
2. Discussion of the Related Art
The DVB-T and DVB-H signal formats are defined in ETSI EN 300 744, “Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television”. DVB-H is further defined in ETSI TR 102 377, “Digital Video Broadcasting (DVB); DVB-H Implementation Guidelines”.
Conventional DVB-T/H diversity receivers are exemplified in the discussion by Yannick Lévy, “DVB-T—A fresh look at single and diversity receivers for mobile and portable reception”, EBU TECHNICAL REVIEW, April 2004, incorporated herein by reference. Additional information regarding DVB-T systems can be found in the following works:                a Mark Massel, “Digital television, DVB-T COFDM and ATSC 8-VSB”, Digitaltvbooks.Com, 2000.        b Seamus O'Leary, “Understanding Digital Terrestrial Broadcasting”, Artech House, 2000.        c Ulrich Reimers, “Digital Video Broadcasting: The International Standard for Digital Television”, Springer, 2001.        d Herve Benoit, “Digital Television: MPEG-1, MPEG-2 and Principles of the DVB System”, Focal Press, 2002.        e Ulrich Reimers. “DVB: The Family of International Standards for Digital Video Broadcasting”, Springer, 2004.        f Walter Fischer, “Digital Television: A Practical Guide for Engineers”, Springer, 2004.        
Briefly, the DVB-T/H system is specified for 8 MHz, 7 MHz, 6 MHz, and 5 MHz channel spacings. The 2K mode and 8K mode are defined for DVB-T and DVB-H transmissions. The 4K mode is defined exclusively for DVB-H transmissions.
The DVB-T/H signal structure is organized into frames. Each frame consists of 68 OFDM symbols. Each symbol consists of a set of carriers: 6817 in 8K mode, 3409 in 4K mode, and 1705 in 2K mode. Each carrier is independently modulated over the duration of each symbol's transmission. All data carriers in one OFDM symbol are modulated using one of the following techniques: QPSK, 16-QAM, non-uniform 16-QAM, 64-QAM, or non-uniform 64-QAM. In addition to transmitted data, the OFDM symbol contains scattered pilot carriers, continual pilot carriers and TPS (transmission parameter signaling). There are 6048 useful carriers in 8K mode, 3024 in 4K mode, and 1512 in 2K mode.
At the transmitter, the modulated carriers in each OFDM symbol are zero padded to the next higher power of 2 and processed by an inverse fast Fourier transform (IFFT) to generate time domain symbols. Each time domain symbol is extended by a guard interval consisting of a cyclic continuation of the useful part of the symbol inserted before it.
In the receiver, the guard interval is removed and the time-domain symbols recovered are converted by a fast Fourier transform (FFT) to recover the frequency-domain symbol. Then the zero padded carriers are removed and the remaining carriers are processed to recover the useful data. This processing includes using the pilot carriers to estimate the transmission channel and removing its effects.
For reception of DVB-T/H signals that have been subjected to time-varying, multipath distortions, channel estimation requires a two-dimensional (2-D) interpolation, which is typically implemented as a one-dimensional (1-D) interpolation in the time domain followed by a 1-D interpolation in the frequency domain. Interpolation in the time domain is a causal process and requires storage of OFDM symbols, typically requiring a large memory.
Prior attempts to reduce these memory storage requirements revolve primarily around performing linear interpolation in the time domain without any extrapolation. See, e.g., Michael Speth et al., Optimum receiver design for OFDM-based broadband transmission—Part II: A case study, IEEE Trans. Communications, vol. COM-49, pp. 571-578 (April 2001). Other solutions have used joint two-dimensional interpolation, which is difficult to implement and still requires large amounts of memory storage. P. Hoeher et al., Pilot-symbol-aided channel estimation in time and frequency, Proc. Sixth Communication Theory Mini-Conf. Conjunction with IEEE GLOBECOM '97, Phoenix, Ariz., pp. 90-96. A collection of methods for performing the time domain interpolation (which do not focus on memory reduction) may be found in Sinem Coleri et al., Channel Estimation Techniquest Based on Pilot Arrangement in OFDM Systems, IEEE Transactions on Broadcasting, Vol 48, No. 3 (September 2002). All of the methods described by Coleri et al. deal with using the same amount of memory, but various ways of performing interpolation.
In severe fading environments, diversity processing is used to improve performance. Signals from multiple antenna ports are combined to obtain improved estimates of the modulated carriers. The lower the correlation between the transmission channels observed at the antenna ports, the more significant the improvement. The downside of conventional frequency-domain combining diversity processing is the duplication in the receive channel hardware. This is especially burdensome for DVB-T/H with its large number of carriers, which require therefore large memory buffers.